1. Field of Invention
The present invention relates to a planar light source, and particularly to a back light module.
2. Description of Related Art
Today, the digital appliances, such as a mobile phone, a digital camera, a digital video camera, a notebook, a personal computer (PC) and a desktop, have been developed tremendously with more convenience, multi-function and attractive outlook. To use such so-called IT (information technology) products, the screen is indispensable for man-machine interface with more convenient maneuver. Recently the liquid crystal display (LCD) has been the major type of screen for most of the mobile phone, the digital camera, the digital video camera, the notebook, the personal computer and the desktop. However the liquid crystal display itself is incapable of emitting light, so that it needs to use a back light module under the liquid crystal display as the light source for display purpose.
FIG. 1 illustrates an exploded drawing of the conventional back light module. In FIG. 1, the back light module 100 includes an optical element 110, a light source 120, a back plate 130, a front frame 140, a plurality of optical films 160 and a reflection sheet 170. Wherein, the light source 120 is disposed approach, including beside or underneath, to the optical element 110. The optical element 110 can be a light guide plate (LGP) or a diffusion sheet. The front frame 140 is used for fixing the optical element 110 and the light source 120 onto the back plate 130. In addition, the back plate 130 supports the optical element 110, the light source 120, the optical films 160 and the reflection sheet 170. In the conventional back light module 100, the optical element 110, the light source 120, the optical films 160 and the reflection sheet 170 are fixed by the front frame 140 and the back plate 130 is used to provide a stable planar light source. Generally, at least one positioning component 150 is usually implemented on the periphery of the back plate 130 for conveniently positioning and assembling optical element 110.
FIG. 2A and FIG. 2B are drawings, schematically showing assembly view and plane view respectively. Here, the optical element 110 includes a main body 112 and a plurality of protrusions 114 wherein two contiguous protrusions 114 and the main body 112 form a positioning groove 116. The positioning component 150 is usually formed by cutting a region of the bottom of the back plate 130 and bending the region up. The positioning component 150 is used to fix the optical element 110 to a position in the back light module to prevent a shifting of the optical element 110 from occurring. In addition, the separate distance gap d between the positioning groove 116 of the optical element 110 and the positioning component 150 is designed as 0.2–0.5 mm approximately for precisely positioning and preventing the optical element 110 from big shift during test or usage. In order to prevent a crack caused by the stress concentration in a corner of the light guide plate during a shock or impact test, it is effective to design a fillet on the corner. But in consideration of practice to open the molding, the fillet radius is larger than 0.7 mm as preferred. Therefore, an excessive small separate distance d is not allowed to form the round fillet at the positioning groove 116 of the optical element 110.
Generally, the impact or shock test must be taken for the liquid crystal display or the liquid crystal panel no matter when in the stage of manufacturing or developing, so that both of manufacturers and customers can evaluate its reliability. However, with the recently lunched liquid crystal display in bigger size, the weight gets larger, accordingly. The stress concentration is more likely occurring at the sharp corners of positioning groove 116 under impact exerted on the positioning component 150, and then causes cracking at the right-angle corner. As a result, the crack on the positioning component 150 causes the poor display quality of the liquid crystal display and people need to handle with such problem by improving design.